Automatic elevator operation requires control of elevator velocity from zero, at the beginning and end of a trip, to speeds therebetween which minimize trip time while maintaining comfort levels and other contraints. The time change in velocity for a complete trip is termed "velocity profile". Automatic elevator control further requires control of the distance travelled during a trip in order to accomplish a precision stop at the destination floor.
Certain velocity profile generation strategies may lead to control instabilities. A common strategy is to use a phase-plane control for precision stopping, wherein dictated velocity is a function of the distance to go to the landing. As the distance-to-go approaches zero, the slope of the velocity/distance curve approaches infinity. Using linear control theory, it can be shown that the slope of the phase-plane curve represents the position error gain for phase-plane control and is proportional to position loop bandwidth. For the speed control loop to track the dictated velocity profile with stability, its bandwidth must be greater by a significant factor than the bandwidth of the position control loop.
One strategy for reducing the required bandwidth is to limit the slope of the phase-plane velocity versus position profile (position error gain) to a maximum value such that the position loop bandwidth is sufficiently lower than the velocity loop bandwidth. Since this theoretically requires an infinite time to reach zero distance-to-go, means must be provided for limiting the run time.
Generally, the torque producing capability of elevator motors may vary with speed due to motor current, voltage, and/or power limitations. If the drive is not capable of maintaining the acceleration limit under all conditions due to these torque limits, some means of reducing the acceleration, and hence torque, in the corresponding portions of the velocity profile must be provided without compromising operation of the drive at its limit or complicating the profile generation more than necessary. The primary problem is that, in order to meet the objectives of precision stopping with torque limiting, the location of the Stop Control Point (SCP) with respect to the destination floor will vary from run to run. (The SCP is that point in the trip where precision stopping maneuvers must be initiated.) In order to calculate the SCP, the deceleration profile must be known.